如何在Binding<CMMotionManager>预览中传递SwiftUI？

Question

我把我的应用程序的motionManager从ContentView传递到SecondView，它运行良好，但是我不知道如何在预览的SecondView中传递它，它需要参数类型Binding<CMMotionManager>

ContentView

import SwiftUI
import CoreMotion

struct ContentView: View {
    
    @State private var motionManager = CMMotionManager()
    let queue = OperationQueue()
    @State private var roll = Double.zero
    
    var body: some View {
        NavigationView {
            VStack {
                
                Text("Roll is: \(roll)")
                    .font(.largeTitle)
                
                NavigationLink {
                    SecondView(motionManager: $motionManager)
                } label: {
                    Text("Change View")
                }
            }
            .onAppear {
                //Detect device motion
                self.motionManager.startDeviceMotionUpdates(to: self.queue) { (data: CMDeviceMotion?, error: Error?) in
                    guard let data = data else {
                        print("Error: \(error!)")
                        return
                    }
                    let attitude: CMAttitude = data.attitude
                    
                    DispatchQueue.main.async {
                        self.roll = attitude.roll
                    }
                }
            }
        }
    }
}

SecondView

import SwiftUI
import CoreMotion

struct SecondView: View {
    @Binding var motionManager: CMMotionManager
    
    var body: some View {
        Text("Hello, World!")
    }
}

struct SecondView_Previews: PreviewProvider {
    static var previews: some View {
        SecondView(motionManager: Binding<CMMotionManager>) <-- //What should i pass here??
    }
}

Answer 1

您不需要对CMMotionManager进行状态/绑定，因为它是引用类型的，正如您自己提到的，它将是单个对象。

所以，只需在常规属性中使用by-ref即可。

struct ContentView: View {
    
    private var motionManager = CMMotionManager()   // << here !!
//...
}

struct SecondView: View {
    let motionManager: CMMotionManager   // << here !!
    
    var body: some View {
        Text("Hello, World!")
    }
}

struct SecondView_Previews: PreviewProvider {
    static var previews: some View {
        SecondView(motionManager: CMMotionManager())    // << here !!
    }
}